Though not code, this is what I specify for areas of light-duty pedestrian Traffic only: Fiberglass-reinforced polyester resin, structurally tested according to SCTE 77 with 3000-lbf (13 345-N) vertical loading.
In sidewalk within 5-ft (1.5-m) of a roadway and unprotected from the roadway SCTE 77 Tier 8 (fiberglass unit with polymer-concrete frame and cover. In driveways, parking lots and similar areas subject to occassional loading by heavy vehicles SCTE 77, Tier 15 (polymer-concrete).
I have them set flush in paved or concrete surfaces. Of course I avoid this whenever possible.